1. Field of the Invention
The present invention relates to a multilayer circuit board which has a predetermined characteristic impedance and on which large scale integrations (LSIs), etc., are put.
2. Description of the Prior Art
Since a common semiconductor device of a beam-lead type or a flip chip type has active elements and/or passive elements within itself, the semiconductor generates heat. Accordingly, a multilayer circuit board on which such semiconductor devices generating heat are put should have the following requirements.
1. Good heat-resisting property.
2. Good heat dissipating property.
3. Obtaining a predetermined characteristic impedance for attaining the impedance matching between multilayer circuit boards and between a multilayer circuit board and the devices put on the board.
4. A small signal propagation delay time.
5. Accurate layer registration.
6. Small electric resistance of conductor layers.
7. A thermal expansion coefficient approaching that of the material, e.g. silicon, of the semiconductor devices generating heat, whereby generation of stress in contacting parts between the multilayer circuit board and the semiconductor devices is avoided.
8. Insulating material chemically stable against an ambient atmosphere and surrounding the conductor layers in an airtight manner.
Taking the above-mentioned requirements into consideration, a multilayer circuit board has been produced by two methods: (1) a laminating method; and (2) a thick film printing method.
In accordance with the laminating method, ceramic material powder, e.g. Al.sub.2 O.sub.3, is mixed with an organic binder and formed into green sheets. Each of the green sheets is punched to make through holes, then printed with metal paste for a conductor circuit. All the green sheets are laminated and then sintered at an elevated temperature of 1350.degree.-1600.degree. C. to produce a multilayer laminated ceramic board. The multilayer laminated ceramic board produced by this method has the following disadvantages.
1. A conductor generates heat and because of this the conductor must be made of a high melting point material, e.g. Mo, W, which has a larger electric resistance than that of Au, Ag or Cu.
2. The signal propagation delay is large because the dielectric constant of the ceramic surrounding the signal conductor layers is high.
3. Thermal resistance is large because, to obtain a predetermined characteristic impedance between a ground conductor layer and the signal conductor layers, the thickness of insulating material (i.e. ceramic) between the ground and signal conductor layers is increased in proportion to the value of the dielectric constant.
In accordance with the thick film printing method, the paste for conductor layers (e.g. paste of Au, Ag or Cu) and the paste for insulating material (e.g. paste of glass, crystalline glass or glass-ceramic) are alternately printed on a fired ceramic plate by a screen process printing method to form an unfired multilayer circuit board. Then, the formed board is fired at a temperature of 700.degree. to 1000.degree. C. to produce a finished article. However, the multilayer circuit board produced by this method has the following disadvantages.
1. Thermal resistance is large because the thermal conductivity of the commonly used insulating material is small.
2. Because the commonly used insulating material has a high dielectric constant, the signal propagation delay is large.
3. The thermal resistance becomes larger because, in order to obtain a predetermined characteristic impedance, a thickness of insulating material between the ground conductor layer and the signal conductor layer is increased in proportion to the value of the dielectric constant.